Electronic module with high cooling power

ABSTRACT

The object of the invention is to improve the cooling of electronic modules ( 40 ), of the type comprising a printed circuit board ( 41 ) contained between two covers ( 42, 42   a,    43 ). The invention relates more specifically to the extraction by means of at least one of the covers ( 42, 42   a,    43 ) of heat produced by at least one component (C 1,  C 2,  C 3 ) borne by the printed circuit board ( 41 ).  
     According to one feature of the invention, the cover ( 42, 42   a, 43 ) responsible for extracting the heat bears at least one device ( 50, 50   a,    50   b,    50   c ) having a high thermal conductivity, making it possible to even out the temperatures of the surface (S 1 ) of the cover ( 42, 42   a, 43 ) with which it is in contact.  
     The invention applies to the electronic modules on board aircraft.

[0001] The invention relates to the cooling of modular electronicdevices, especially but not exclusively to “on board” devices placed onboard aircraft, or tanks, etc.

[0002] “On board” devices must operate under harsh environmentalconditions (vibrations, shocks, humidity, accelerations, heatdissipation difficulty).

[0003] These difficult conditions may cause breakdowns. This means thatthese electronic units have to be designed for the purpose of makingtheir repair and their maintenance easier.

[0004] One of the new aeronautical standards stipulates that they beproduced in the form of a modular structure called LRM (Line ReplaceableModule), such structures being installed in racks.

[0005] A rack may include a large number of electronic modules. Thismakes rapid maintenance, handling and repair work easy, but tends tomake it more difficult to extract the heat produced by the variouscomponents contained in the modules. These components, of increasingsophistication, produce increasing amounts of heat. The inventions aimsto solve the problem of extracting this heat.

[0006]FIG. 1 shows in perspective a rack 1 containing several electronicmodules. The rack 1 is generally parallelepipedal, with a front face 3for inserting and extracting the electronic modules 2 a to 2 e and fiveother faces 4, 5, 6, 6 a, 7. The face 7 consists of a back plate whichcloses off the rack 1 and carries connectors into which thecorresponding connectors of the modules 2 a to 2 e are plugged.

[0007] The bottom and top plates 4, 5 have slideways, for example madeof metal, respectively 9, 9 a, for guiding the electronic modules 2 a to2 e in the rack 1 and for keeping them therein.

[0008] The rack 1 may include openings for air circulation, for thepurpose of cooling the electronic modules 2 a to 2 e. The cooling air(shown symbolically by arrows labeled 30) is generally injected into therack via the bottom plate 4, through openings 32 called air inletopenings located between the slideways 9. The cooling air 30 circulatesbetween the modules 2 a to 2 e (and possibly in these modules). Theheat-laden air 33 leaves the rack 1 through outlet openings 34, formedbetween the slideways 9 a and the top plate 5.

[0009]FIG. 2 shows in a simplified manner, in cross section, an exampleof a conventional electronic module structure, as may be found, forexample, in patent GB-A-2 270 207.

[0010] The electronic module 2 e comprises two covers, 18, 19, forexample made of aluminum, having a thickness of the order of 1millimeter. Aluminum is a particularly advantageous material from thestandpoint of on board mass, in that it has both a low density and avery good mechanical rigidity even with a small thickness; it also has amoderate thermal conductivity, although very much less than that ofcopper.

[0011] A printed circuit board 15 is placed in the space bounded by thetwo covers 18, 19. It is gripped around its periphery by the edges ofthe covers. The printed circuit board 15 bears the various components ofan electronic circuit, among which at least one component 22 isproducing a large amount of heat in operation (for example a powerfulmicroprocessor).

[0012] In the example in FIG. 2, the component 22 has a thickness e1which corresponds approximately to a distance d1 between the inside ofthe first cover 18 and that face (face 23) of the printed circuit whichbears the component. The component 22 is thus in practice directly incontact with the first cover 18 via, for example, a thermal interface ormatching layer 25 which promotes thermal contact and is electricallyinsulating (an elastomer or epoxy resin).

[0013] Thus, the covers 18, 19, and particularly the first cover 18 inthe example shown, provide, in addition to their function of protectingthe printed circuit 15 and of electrostatic shielding, a heat sinkfunction.

[0014] The heat transmitted to the cover is itself extracted byconduction to the slideways 9, 9 a of the rack and then to the walls ofthe rack; the heat is also extracted by convection, thanks to thecooling air 30 coming from the abovementioned ventilation. Ventilationinside the electronic modules 2 a to 2 e may be added, by providingbottom and top openings 26, 27 in the bottom and top walls 18 a, 18 band 19 a, 19 b of the covers, respectively.

[0015] Even when the three means mentioned above are combined, theextraction of heat may be insufficient with current components. One ofthe aims of the present invention is to improve this extraction.

[0016] For this purpose, the invention provides an electronic module ofthe type intended to operate in a rack, comprising at least one printedcircuit board, at least one protective cover defining a housing for theboard, at least one component producing heat in operation and mountedvia its bottom face on the board, and a thermal link between a top faceof the component and the cover, characterized in that the thermal linkcomprises, interposed between the top face of the component and thecover, a device having a high thermal conductivity, this device havingan area greater than that of the component and having a thermalconductivity greater than that of the cover.

[0017] The word “interposed” should be understood to mean that thedevice is at least partly placed in series between the component and thecover in the thermal circuit of heat flow from the component to thecover. This even applies if the device having a high thermalconductivity also includes a part which extracts the heat directly tothe natural air or to the forced circulation air which lies outside thecover. However, in a preferred embodiment, the device having a highthermal conductivity is physically interposed in its entirety betweenthe component and the cover and is applied over its entire area (whichis greater than that of the component) against this cover.

[0018] The term “device having a high thermal conductivity”, or in short“HTC device”, should be understood to mean any device or element havinga better thermal conductivity than that of the material of which a coverresponsible for extracting the heat is made, so as to make it possibleto even out or tend to even out the temperature at all points on asurface of the cover with which this HTC device may be brought intocontact. They may, for example, when the cover is made of aluminum, beelements based on one or more materials whose thermal conductivity isgreater than or equal to that of copper, or else devices which involvechanges of phase of a solid, liquid or gaseous element allowingsubstantial amounts of energy to be transported; some of the latterdevices are known especially as “heat pipes” and may consist of a hollowplate containing a liquid, the cooling relying on the energy consumed bythe liquid-to-gas phase change in a closed circuit in the hollow plate.In this case, it will be understood that one speaks of an equivalentthermal conductivity, corresponding to the heat extraction capability:the equivalent conductivity is that of an imaginary material which,having the dimensions of the device (for example the heat pipe), wouldhave the same heat extraction capability.

[0019] The application of an HTC device to a region of a cover makesthis region better able to conduct heat and, as it were, better able todistribute it to the rest of the cover which, although a poorer thermalconductor, then benefits from a larger area conducive to thisconduction. This results overall in an increase in the thermalconductivity of the cover, in a ratio very much greater than that of theincrease in the mass of the cover resulting from the presence of the HTCdevice.

[0020] A better understanding of the invention, and further features andadvantages that it affords, will appear on reading the description whichfollows, given as nonlimiting example with reference to the appendedfigures, among which:

[0021]FIG. 1, already described, shows a rack of a known type containingconventional electronic modules;

[0022]FIG. 2, already described, shows, in a sectional view, thestructure of an electronic module illustrated in FIG. 1;

[0023]FIG. 3 is a sectional view of an electronic module according tothe invention;

[0024]FIG. 4 is a sectional view of an electronic module in anotherembodiment of the invention;

[0025]FIG. 5 illustrates, in a sectional view, an embodiment of a devicehaving a high thermal conductivity according to the invention, placed ona cover shown in FIG. 3;

[0026]FIG. 6a is a sectional view showing a cover illustrated in FIG. 5,in which an opening facing a device having a high thermal conductivityis made;

[0027]FIG. 6b is a sectional view showing the cover and the devicehaving a high thermal conductivity illustrated in FIG. 6, but with thelatter being positioned differently;

[0028]FIG. 6c is a sectional view similar to FIG. 6a and furthermoreillustrating that the device having a high thermal conductivity isprovided with cooling fins; and

[0029]FIG. 7 is a sectional view of the cover illustrated in FIG. 5,showing an embodiment of the invention in which a device having a highthermal conductivity forms an integral part of the cover.

[0030]FIG. 3 shows an electronic module 40 according to the invention,in a sectional view similar to that of FIG. 2.

[0031] The module 40 comprises a printed circuit board 41 between twocovers 42, 43, for example made of aluminum, similar to those of FIG.22.

[0032] The board 41 bears components among which are the dissipativecomponents C1, C2, C3 distributed here over both faces 45, 46 of theboard.

[0033] According to the invention, one (or possibly more) devices havinga high thermal conductivity, or HTC devices, 50, 50 b are interposed inseries (in the direction in which the heat is extracted from thecomponent to the cover) between the top face of certain components (thatface opposite the bottom face turned toward the board) and the cover.

[0034] A material having a relatively good thermal conductivity(although very much less than that of the HTC device) may fill the gapbetween the component and the HTC device 50 or 50 b when the height ofthe component and the shape of the casing and of the HTC device allowonly the top face of the component to touch the HTC device at the sametime as the HTC touches the cover. The materials or products mostcommonly used to provide this thermal link are, for example, elastomersfilled with heat-conducting particles, resins, greases, adhesives, gelsand surface treatments. It should be noted that sheets of thephase-change type now exist which consist of a support film bearing acoating which changes state at a given temperature, thereby allowing themicroscopic irregularities to be filled and the thermal contact to beimproved.

[0035] The function of the HTC device 50 is especially to even out or atleast greatly reduce the temperature differences presented by a surfacewith which it is in contact. For this purpose, with the first cover 42(but also the second cover 43 in the example) being made from aluminum,the HTC device 50 may, for example, be made of copper. However, the HTCdevice may also be made of any other material having a thermalconductivity greater than that of the material from which the coverbearing it is made; it may be considered that the implementation of thedevice according to the invention becomes greatly advantageous when itsthermal conductivity is greater than or equal to 1.5 times that of thematerial of which this cover is made (or from which the latter is made).

[0036] Among materials possessing a thermal conductivity even greaterthan that of copper, mention may be made, for example, of materialsbased on high-conductivity graphite, especially pyrolitic graphite, orcomposites of the carbon/carbon type, or else materials of the typehaving a structure formed by a substrate with a coating of diamond.Finally, it should be noted that the HTC device 50 may also be made froma structure employing the phase change of a liquid, solid or gaseouselement, as already mentioned above and as will be explained further inthe continuation of the description.

[0037] The HTC device 50 is in the form of a plate whose thickness e5,for example between 1 and 4 millimeters, is chosen to be greater thehigher the thermal power to be extracted.

[0038] The HTC device 50, that is to say the HTC plate 50, is preferablydesigned to have a surface S1 of larger area than that of the surface S2of the component. Tests have given a very satisfactory result with anarea of surface S1 of the HTC plate 50 of about 30 cm² (for an area ofS2 of around 8 cm²) and a thickness e5 of around 1.5 millimeters. Thesedimensions of the HTC plate are given solely by way of indication and donot constitute a limiting example—they may be tailored to the dissipatedpower levels; in addition, it should be noted that a reduction inthickness e5 of the HTC plate may be compensated for by an increase inits area and vice versa.

[0039] It is possible to further increase the effectiveness of the heatextraction by giving the area of the surface S1 of the HTC plate 50 anelongate shape and by orienting it so that its largest dimension is inthe direction most favorable to cooling it. Its largest dimension may,for example, be approximately parallel to the flow of cooling air 30, orparallel to the height H of the electronic module if the slideways 9, 9a constitute a cold source, or else it may be oriented differently,especially according to tests and/or a particular configuration.

[0040] The HTC plate 50 may be made and mounted directly (as shown inFIG. 3) on the cover 42, in the factory, that is to say in the sameindustrial step as that for manufacturing the cover 42. This allowsexcellent thermal contact to be achieved. If the HTC plate 50 has to beattached to the cover 42 after the latter has been manufactured, it maybe advantageous to do this by means of a layer referred to as a thermalinterface (not shown) in order to guarantee the quality of the contact.

[0041] Of course, it is possible to place, if necessary, several HTCdevices 50, 50 a, on the same cover 42, 43.

[0042] The second and third components C2 and C3 mounted on the secondface 46 illustrate another version of the invention, in which a singleHTC device or plate, labeled 50 b, serves to promote the extraction ofthe heat produced by several components. The differences in height ofthe components C2 and C3 are compensated for by the fill materialinterposed between the top face of the component and the HTC device 50b.

[0043] For example, the HTC plate 50 b in this configuration has an areaof surface S5 of around 42 cm², which allows the heat produced by thetwo components C2, C3 to be extracted with an efficiency comparable tothat described above in regard to component C1.

[0044]FIG. 4 shows an electronic module 40 a in the configuration thatit would have if the heat were to be extracted from the bottom face ofthe components.

[0045]FIG. 5 is a sectional view of an HTC device, labeled 50 c, of thetype referred to above as a “heat pipe”, employing the effect of a phasechange in an element, for example a liquid, for example water. The HTCdevice 50 c may be used on either of the covers 42, 42 a, 43. In theexample, it is shown mounted on the first cover 42 (shown partly in FIG.5) at the same position, for example, and with the same function as thatin the case of the HTC device 50 shown in FIG. 3.

[0046] The HTC device 50 c is therefore mounted on the inner face 42 iof the first cover 42. It is in a form similar to that of the HTC plate50 with, for example, the same length L3 (parallel to the height H ofthe cover) but, however, with a thickness e5 may be greater, for examplearound 3 millimeters, especially depending on the machining means used.In the nonlimiting example of the description, it has two parallelcommunicating channels 60, 61 which extend parallel to the plane of thecover 42 and which constitute a closed circuit. On these two channels,the first channel 60 is in contact with a hot wall 62 intended toreceive heat delivered by the first thermal link 47 illustrated in FIG.3. The second channel 61 is in itself in contact with a cold second wall63 which is contact with the cover 42.

[0047] Under these conditions, a certain quantity of water contained inthe first channel 60 passes into the vapor phase when it is heated bythe first wall 62 and it passes in vapor form (shown symbolically by anarrow 64) into the cooler second channel 61, where the vapor condensesand then returns in water form 65 to the first channel 60.

[0048] The closed circuit consisting of the two channels ormicrochannels 60, 61 may be easily produced on an industrial scale incopper or any other material having a good thermal conductivity. Thesetwo channels may be formed by narrow grooves machined, for example, incopper plates 66, 67 separated by an intermediate plate 68 and closed byend pieces 69 a, 69 b. Narrow grooves, for example 1 or 2 millimeters,for closing the channels, allows several such closed circuits to bereproduced, in parallel, in the dimensions allocated to the HTC device50 c.

[0049] The very high heat transfer capacity that phase-change systemssuch as the HTC device 50 c possess allows them to create a homogeneousand substantially uniform temperature distribution over the entiresurface of the cover 42 to which they are applied.

[0050] The increase in the mass of a cover 42, 42 a, 43 by the presenceof an HTC device may be compensated for by making, in this cover, anopening facing at least one HTC device, as shown in FIG. 6a.

[0051]FIG. 6a shows schematically, and partly, a cover such as, forexample, the first cover 42, in a sectional view similar to that of FIG.5. The cover 42 bears any one of the HTC devices described above, forexample the first HTC device 50 illustrated in FIG. 3. In the exampleshown, the HTC device 50 is mounted on the inner face 42 i of the cover42. According to one feature of the invention, the cover 42 has one (ormore) openings 88 formed opposite the HTC device 50. The opening 88 mayhave dimensions (of which only the length L5 is shown) slightly smallerthan those of the HTC device (of which only the length L3 is shown) sothat the HTC device closes the opening 88. Part of the HTC device is indirect contact, outside the cover, with the external air. However, theHTC device is thermally linked to the cover 42 at least around theperiphery of the opening 88, so that the cover for its part contributesto extraction of the heat from the component. Here again, the surfacearea of the HTC device is greater than that of the component with whichit is in contact (preferably direct contact).

[0052] The nonlimiting example in FIG. 6a shows a case in which the HTCdevice 50 is attached to the cover 42, after the latter has beenmanufactured, and in the example it is fixed by screws 89 to the innerface 42 i of this cover.

[0053]FIG. 6b is a view of the cover 42 similar to that of FIG. 6a, alsoshowing an opening 88. But in the case of FIG. 6b, the HTC device 50 isattached to the cover 42 from the outside, that is to say it is fixed tothe outer face 42 e of this cover. The HTC device 50 may thus beattached to the cover 42 from the outside or else from the inside (FIG.6a), especially for reasons of ease of removing the cover.

[0054]FIG. 6c is a view of the cover 42 which differs from that of FIG.6a solely by the fact that the HTC device 50 includes cooling fins 90.The HTC device 50 is fixed to the inner face 42 i of the cover 42 andthat part S1 a of its surface S1 which opens to the outside, by virtueof the opening 88, is provided with fins 90 which tend to promote itscooling.

[0055]FIG. 7 is a sectional view of a cover, for example the cover 42,and it shows another way of placing an HTC device 50. In this version,the HTC device 50 is placed in the extension of the plate which formsthe cover and it forms an integral part of the cover; the HTC device mayeither be obtained directly by machining or, for example, be producedseparately and fastened to the cover by welding, for example after ithas been fitted into an opening (not shown) made in this cover. It willbe understood that, in this case, the device is considered to beinserted at least partly in series between the component and the cover,via its periphery welded to the cover.

[0056] Of course, in this latter version of the invention, the HTCdevice 50 may optionally be provided with fins 90 (not shown in FIG. 7),optionally over its entire surface S1.

1. An electronic module of the type intended to operate in a rack (1),comprising at least one printed circuit board (15), at least oneprotective cover (42) defining a housing for the board, at least onecomponent (C1) producing heat in operation and mounted via its bottomface on the board, and a thermal link between a top face of thecomponent and the cover, characterized in that the thermal linkcomprises, interposed between the top face of the component and thecover, a device (50) having a high thermal conductivity, this devicehaving an area (S1) greater than that (S2) of the component and having athermal conductivity greater than that of the cover.
 2. The electronicmodule as claimed in claim 1, characterized in that the device having ahigh thermal conductivity consists of a plate of material of greatthermal conductivity.
 3. The electronic module as claimed in claim 2,characterized in that the device having a high thermal conductivityconsists of a hollow plate of the heat-pipe type, operating on theprinciple of a change of phase, in a closed circuit, of a liquidcontained in the hollow plate.
 4. The electronic module as claimed inclaim 1, characterized in that the device (50, 50 a, 50 b, 50 c) havinga high thermal conductivity has a thermal conductivity equal to orgreater than 1.5 times that of the material from which the cover (42, 42a, 43) bearing it is made.
 5. The electronic module as claimed in one ofthe preceding claims, characterized in that at least one device (50, 50a, 50 b, 50 c) having a high thermal conductivity has an elongate shapeso as to have a larger dimension (L3), in order to allow this largerdimension (L3) to be oriented the most favorably for cooling the device(50, 50 a, 50 b, 50 c) having a high thermal conductivity.
 6. Theelectronic module as claimed in one of the preceding claims,characterized in that a cover (42, 43) bearing a device (50, 50 a, 50 b,50 c) having a high thermal conductivity has an opening (88) formedsubstantially opposite the device having a high thermal conductivity. 7.The electronic module as claimed in any one of claims 1 to 6,characterized in that a device (50) having a high thermal conductivityforms an integral part of a cover (42, 42 a, 43).